Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by application of a transparent or clear top coat over the base coat have become increasingly popular as original finishes for automobiles. The color-plus-clear systems have outstanding appearance properties such as gloss and distinctness of image due in large part to the clear coat.
Clear film-forming compositions, particularly those used to form the transparent top coat in color-plus-clear systems for automotive applications, are subject to damage from numerous environmental elements. Such elements include acidic precipitation, exposure to ultraviolet radiation from sunlight, high relative humidity and temperatures, and defects due to impact with small, hard objects resulting in chipping and scratching of the coating surface.
Typically, a harder more highly crosslinked film may exhibit improved scratch resistance, but it is much more susceptible to chipping and/or thermal cracking due to embrittlement of the film resulting from a high crosslink density. A softer, less crosslinked film, while not prone to chipping or thermal cracking, is susceptible to scratching, water spotting and acid etch due to a low crosslink density of the cured film.
U.S. patent Ser. No. 08/904,597, filed Aug. 1, 1997 discloses curable compositions based on functional polysiloxanes, particularly hydroxyl functional group-containing polysiloxanes, which are suitable for use as clear coats in color-plus-clear systems for automotive applications. The functional group-containing polysiloxanes provide clear coatings with, inter alia, improved mar and acid etch resistance.
U.S. Pat. No. 5,853,809 discloses clear coats in color-plus-clear systems which have improved scratch resistance due to the inclusion in the coating composition of surface reactive inorganic microparticles such as colloidal silicas which have been modified with a reactive coupling agent. There, nonetheless, remains a need in the automotive coatings art for top coats having improved initial scratch resistance as well as enhanced post-weathering ("retained") scratch resistance without embrittlement of the film due to the high crosslink density.